Cabbage-based slaws — cole or otherwise — are transcendent when properly executed, but so easy to mess up completely. No one wants a bowl of cabbage shreds swimming in overly-sweetened, watered-down mayo, and — luckily — no one has to resign themself to that fate. To keep your slaw from sogging out, you just need a little salt.
Sodium chloride (the salt we eat), is a pretty good solute, which means it has a strong tendency to draw water out of fruits and vegetables through their cellular walls. It’s just up to you to decide if you want that process (called osmosis) to happen before or after you dress your slaw. (The salt and sugar in your dressing are quite capable of drawing out moisture.) This isn’t such a big decision if you are making and then eating your slaw immediately, but if you plan to take it to a potluck the next day, or even delay your consumption of it for a little while, you want to get that excess moisture out of there before it sees even a spoonful of mayo (or any other dressing).
Luckily, this manoeuvre is easy to pull off; A.A. Newton explained how to do it in her larger guide to coleslaw, but all you need to do is toss 1/4 of a teaspoon of table salt with half a head of shredded cabbage (and any other vegetables you want to add, like carrots or onions), then let it rest for about ten minutes on a baking sheet lined with paper towels, or in a colander. Next, remove the excess moisture by blotting with more paper towels, or place the mixture in a salad spinner and spin like heck. Now you can dress your slaw and eat it, or store the cabbage shreds in a clean, covered bowl for up to 24 hours. (If any more liquid seeps out of the shreds, just blot it away.)